1. Field of the Invention
The present invention pertains to a method for direct preparation of solid nonmetallic particulates by liquid comminuting from molten material introduced into another liquid which may be moving, the method being particularly useful with oxidizing salts of ammonia.
2. Description of the Prior Art
Prills or small, solid spherical particles are used for a variety of purposes as for application of fertilizers and for inclusion of oxidizing materials such as ammonium dinitramide (ADN) or ammonium nitrate (AN) in energetic compositions. In the prior art, prills are made by spraying molten material through a number of nozzles to form droplets and allowing the droplets to fall through a column of air or liquid until they cool sufficiently to solidify and can withstand impacting the bottom of the column without distortion or damage.
This method has a number of deficiencies For one, the height of such cooling columns using air typically ranges from 20 or 30 feet to nearly 100 feet in height which is not a function of volume of production and thus cannot be scaled down for low volume or laboratory production. For another, precise control of prill sizes, particularly in small sizes such as the about 50 microns desirable with oxidizing materials, is not possible. Further, precise control of the cooling rate is not possible so that the product has irregular crystallization or fractures caused by either too slow or too rapid a cooling rate. Also, impacts of droplets or incompletely hardened particles with other droplets, particles, or column sides results in a product with indentations and other irregularities. Similar problems occur in cooling droplets in liquid which boils or otherwise changes phase or releases gas on contact with the molten droplets.
Further, this method is dangerous with an energetic material such as the oxidizing salts of ammonia, ADN and AN, since a relatively large quantity of the material is melted together, as in a pan with a liquid level providing sufficient head for spraying, and the hazard of fire or violent reaction increases with temperature and volume of material. A fire or violent reaction may, of course, propagate from the heated material to cooler material awaiting melting or already prilled. The dangers of prior art prilling methods and apparatus are particularly serious with ADN which is extremely sensitive although having a nominal freezing and melting temperature of about 93.degree. C. substantially below that of AN where this temperature is about 175.degree. C. In a conventional prilling tower, where hot droplets or particles may contact causing ignition or violent reaction which may then propagate, the sensitivity of ADN, which detonates in small amounts, could be disastrous. Therefore, the use of ADN has been limited by its unavailability in small, regular particles of uniform size since grinding of this material even when cool, unlike AN, is also likely to result in fire or violent reaction.
Even in the absence of such reactions, ADN is also difficult to handle, for prilling or other purposes, because molten ADN is highly corrosive to all metals. Also, ADN can supercool as much as 70.degree. C. between the molten and solid states, thus requiring the removal of large quantities of heat to insure solidification of prills. Even if it were safe to prill ADN in air columns, this supercooling property would require large and expensive columns.
Insofar as known to applicants, prilling of ADN in liquid filled columns has not heretofore been attempted since ADN is highly soluble in water and hot ADN presents a significant safely hazard with oxidizable materials such as mineral oils.
Other problems involve the need for prills to include stabilizers to prevent decomposition or crystalline phase change and to eliminate moisture even when in the unprocessed material to be prilled. It is apparent that prilling in an air column introduces atmospheric moisture; also, certain stabilizers are formed by water releasing chemical reactions.